In a large pan or dutch oven combine the brown sugar, butter, corn syrup, and salt. Bring to a boil, and let it continue to boil for 5 minutes. Remove from the heat and stir in the baking soda quickly.Pour the brown sugar mixture over the popcorn and mix well.Divide the popcorn among 3-4 large baking pans. Put in the oven and stir every 15 minutes. After an hour take it out of the oven and let it cool.